The degree of integration of integrated circuits has recently become much higher, and the photolithography for superfine patterns having a line width of half micron or below is required in the production of semiconductor substrates for VLSI and the like. In order to meet such a requirement, a wavelength used in an exposure apparatus for photolithography becomes shorter, and nowadays the use of far ultraviolet light in a shorter wavelength region, or an excimer laser beam (e.g., XeCl, KrF, or ArF) is investigated.
A chemical amplification resist is known to be used in the pattern formation by the photolithography in such a wavelength region.
In general, the chemical amplification resist is roughly divided into three types, the so-called two-component, two point five-component and three-component types. The chemical amplification resist of two-component type comprises a combination of a compound capable of generating an acid by photolysis (hereinafter referred to as a "photo-acid generator" sometimes) with a binder resin. The binder resin combined with such a compound is a resin having in its molecule, a group capable of being decomposed by the action of an acid to increase the solubility of resin in an alkaline developing solution (which is hereinafter referred to as an "acid-decomposable group" sometimes). The resist of two point five-component type further contains a low molecular weight compound having an acid-decomposable group in addition to the above described two components. The resist of three-component type contains a photo-acid generator, an alkali-soluble resin and the low molecular weight compound described above.
Although the chemical amplification resist described above is a photoresist suitable for exposure to ultraviolet light or far ultraviolet light, it is yet necessary for the resist to meet requirements for characteristics from the viewpoint of practical use. In the case of using a KrF excimer laser beam of 248 nm as exposure light, for instance, resist compositions comprising a hydoxystyrene polymer showing only slight absorption of the laser beam into which acetal or ketal groups are introduced as protective groups are proposed, e.g., in JP-A-2-141636, JP-A-2-19847, JP-A-4-219757 and JP-A-5-281745. In addition, compositions which are similar to the above, except that tert-butoxycarbonyloxy or p-tetrahydropyranyloxy groups are introduced thereinto as acid-decomposable groups, are proposed, e.g., in JP-A-2-209977, JP-A-3-206458 and JP-A-2-19847.
Although those compositions are suitable for the case of using the KrF excimer laser beam of 248 nm, they have low sensitivity to an ArF excimer laser beam because their absorbance at the wavelength of ArF excimer laser beam is substantially high. This drawback is accompanied with other drawbacks of causing deterioration, e.g., in definition, focus latitude and pattern profile. Therefore, it is necessary for those compositions to be improved in many points.
As photoresist composition suitable for the ArF excimer light source, combinations of (meth)acrylic resins showing less absorption of light than partially hydroxylated styrene resins with a compound capable of generating an acid upon exposure are proposed, e.g., in JP-A-7-199467 and JP-A-7-252324. Among others, JP-A-6-289615 discloses an acrylic resin having carboxyl groups the oxygen atoms of which are bonded to tertiary carbon-containing organic groups to form ester linkages.
Further, JP-A-7-234511 discloses an acid-decomposable resin containing both acrylate and fumarate as repeating units. However, those resins are insufficient in view of pattern profile and adhesion to a substrate. Thus, it is true that satisfactory resist properties are not obtained.
Furthermore, a resin into which an alicyclic hydrocarbon moiety is introduced is proposed for the purpose of improving dry etching resistance. However, the introduction of alicyclic hydrocarbon moiety is accompanied with a problem in that a system including the resin becomes extremely hydrophobic, and it is difficult to conduct development with an aqueous solution of tetramethylammonium hydroxide (hereinafter referred to as TMAH sometimes) which has hitherto been widely employed as a developing solution of resist. Thus, a phenomenon of peeling the resist from a substrate during the development is observed.
In response to the increase in the hydrophobic property of resist, an attempt has been made that an organic solvent such as isopropyl alcohol is added to the developing solution. Although the result is recognized to a certain extent, other problems, for example, fear of swelling the resist film and complexity of the process arise. Thus, the problem is not always solved.
As to an approach to improvement of the resist, many attempts of compensation for the hydrophobic property of alicyclic hydrocarbon moieties by the introduction of hydrophilic groups have been made.
In general, a method of copolymerizing a monomer having a carboxylic acid group such as acrylic acid or methacrylic acid together with a monomer having the alicyclic hydrocarbon moiety is adopted. Although the adhesion to a substrate tends to be improved as a result of the introduction of carboxylic acid group, such a method has problems in that the dry etching resistance is deteriorated and in that the reduction in a thickness of resist film is remarkably observed, and thus the above described problems still remain. Further, a method of copolymerizing a monomer having a hydroxy group or a cyano group such as 2-hydroxyethyl methacrylate (HEMA) or acrylonitrile in place of the monomer having a carboxylic acid group together with a monomer having the alicyclic hydrocarbon moiety is proposed in order to solve the problems in a developing property as described in JP-A-7-234511. However, the method is still insufficient for attaining the purpose.
Recently, investigations on improvement in the adhesion to a substrate by using a lactone structure such as mevalonic lactone have been made as described in Journal of Photopolymer Science and Technology, Vol. 9, pages 509-522 (1996) and Journal of Photopolymer Science and Technology, Vol. 10, pages 545-550 (1997). However, since these methods are inferior in the developing property, an overcoat layer which makes the process complex is necessary for maintaining the good developing property. Further, the adhesion to a substrate is still insufficient according to these methods.
Also, monomers having both a carboxylic acid group and the alicyclic hydrocarbon moiety in their molecules are investigated in order to improve the dry etching resistance, developing property and adhesion to a substrate as described in JP-A-8-259626, Journal of Photopolymer Science and Technology, Vol. 9, pages 377-385 (1996) and Journal of Photopolymer Science and Technology, Vol. 9, pages 447-456 (1996). Due to extremely high solubility of the monomer in a developing solution, to use the monomer is attended with other problems of decrease in the adhesion to a substrate and lack of an aptitude to a standard developing solution.
Moreover, an attempt of improvement has been made by copolymerizing an alicyclic hydrocarbon monomer having an alcoholic hydroxy group as described in Journal of Photopolymer Science and Technology, Vol. 10, pages 561-570 (1997) with the above-described monomer. The attempt, however, has not reached the satisfactory level.
Besides the introduction of alicyclic hydrocarbon moiety into a side chain of an acrylate monomer as described above, a method for imparting the dry etching resistance utilizing a polymer having the alicyclic hydrocarbon moiety in the main chain thereof is investigated. Since the method is accompanied with the above described problems, improvements using methods similar to those described above are further investigated.
For instance, introduction of hydroxy group into the main chain of a norbornene polymer for the purpose of imparting the adhesion to a substrate is proposed as described in Journal of Photopolymer Science and Technology, Vol. 10, pages 529-534 (1997) and Journal of Photopolymer Science and Technology, Vol. 10, pages 521-528 (1997). However, satisfactory results are not obtained with respect to both the developing property and the adhesion to a substrate.
Further, in SPIE, Vol. 3049, pages 92-105 (1988), a composition containing a polymer obtained by a ring-opening polymerization of a norbornene ring or a polymer containing a norbornene ring in the main chain thereof, and each having a carboxy group and a tert-butyl ester group is described. However, the composition is defective in that the adhesion to a substrate and the aptitude to a standard developing solution are practically insufficient.
A composition containing a polymer obtained by a ring-opening polymerization of a norbornene ring or a polymer containing a norbornene ring in the main chain thereof, and each having an acid-decomposable group and a carboxy group is described in EP-A-789,278. Also, a photoresist composition comprising a resin obtained by polymerization of a monomer containing a norbornene ring having an acid-decomposable group is described in WO 97/33198. Further, the photoresist composition further containing a carboxylic acid is described in JP-A-9-274318.
For the chemical amplification resist, the alkali solubility of resin is an important factor which relates to the resolution and developing property thereof, and control of the alkali solubility is conducted by determining a content of a carboxy group attached to its side chain. However, the photoresist composition containing the resin described above has a problem in that the developing property and the adhesion to a substrate are not satisfactory.
In JP-A-10-55069, there is described a radiation-sensitive resin composition containing a resin which is decomposed by the action of an acid to increase its solubility in an alkaline developing solution and has a group represented by --X--COOH (wherein X represents a specific divalent group) at least at one terminal of the molecular chain thereof and a photo-acid generator which generates an acid upon irradiation of light, which has good transparency to far ultraviolet light and excellent adhesion to a substrate as well as excellent sensitivity, resolution and developing property.
The above described composition for exposure to short wavelength light has a problem in that a solution (coating solution) of the composition prepared by dissolving the composition in an organic solvent has poor storage stability to generate aggregates or the like. Thus, further improvements have been desired.